A mainspring is a spiral torsion spring of metal ribbon used as a power source in mechanical watches, storing energy when wound and releasing it to drive the movement.
The mainspring is the coiled metal ribbon that stores and releases energy to power a mechanical watch movement. Wound manually or by a rotor (automatic), it stores potential energy as it coils. As it gradually unwinds, it drives the gear train, which ultimately moves the hands. Mainsprings are typically made from high-grade steel alloys or modern materials like Nivaflex, designed for consistent tension throughout their release.
Mainsprings release energy unevenly — strong when fully wound, weaker as they run down. This variation in torque causes the balance wheel to oscillate at slightly different amplitudes, affecting accuracy. Modern designs mitigate this through barrel design, remontoir mechanisms, and automatic rewinding. Some movements include a 'Geneva stop' or limiting device to use only the most consistent middle portion of the mainspring's power.
Modern mainsprings are extremely durable and rarely break with proper maintenance. However, the alloy can fatigue over decades, reducing elasticity and power reserve. Regular servicing (every 5-7 years) includes cleaning, lubricating, and inspecting the mainspring. If cracked, set, or corroded, replacement is necessary. Antique watches with original mainsprings may have reduced power reserves or risk of breakage during winding.